Process and apparatus for gas purification



y 1959 J. WUCHERER ETAL 2,895,304

PROCESS AND APPARATUS FOR GAS PURIFICATION 7 Filed May 31, 1956 United States Patent PROCESS AND APPARATUS FOR GAS PURIFICATION Johannes Wucherer, Munich, Hermann Linde, Munich- Solln, and Fritz Jakob, Munich, Germany, assignors to Gesellschaft fiir Lindes Eismaschinen Aktiengesellschaft, Hollriegelskreuth, near Munich, Germany, a German company Application May -31, 1956, Serial No. 588,372 Claims priority, application Germany June 2, 1955 '16 Claims. (CI. 62-13) 1 The invention to be hereinafter described relates to a process for separating compressed gas mixtureswith a high content of hydrogen, more especially for the purpose producing hydrogen-nitrogen mixtures for the syn thesis of ammonia, the gas mixture being subjected to intense cooling and fractional condensation, and finally to a washing treatment with liquid nitrogen.

In order to be able to carry out this separation, the hydrogen-containing crude gas is compressed in known manner to a comparatively high pressure, low-cooled at this pressure and finally washed at the final pressure of the compression with a nitrogen which has been obtained from an air separation plant. Since the latter only supplies nitrogen at atmospheric pressure, the nitrogen must be initially heated, then compressed again to a higher pressure, condensed and supplied to a nitrogen washing tower for washing out the impurities which boil at lower temperature. The cold required for meeting the cold losses is obtained by throttle expansion of high-pressure nitrogen.

The washing with liquid nitrogen in the manner described has the advantage that not only carbon monoxide and oxygen, but all constituents boiling at a temperature higher than that of nitrogen are reliably and very extensively separated out. Since this washing must, however, take place at the temperature of the boiling nitrogen, that is to say, in the region of 190, quite complicated equipment is required therefor. Not only must the hydrogen-containing gas be completely freed from steam and carbon dioxide and cooked in a plurality of heat-exchangers to low temperature, but it is also necessary to have a high-pressure nitrogen compressor and a set of heat-exchangers for this nitrogen, as well as an ammonia refrigerating machine with a condenser or an expansion machine and a series of gas coolers. The high-pressure nitrogen, which produces the degree of coldness necessary for the process by throttle expansion to somewhat above washing tower pressure, and is thereafter liquefied, serves as a washing agent for the synthesis gas.

With the usual process, it is true that the high-pressure nitrogen, owing to its great specific heat, permits substantially more dissociation products to be conducted in counter-current thereto in the cold exchange than conforms to its actual amount. Consequently, it is also possible for dissociation products of the crude gas to be warmed therewith. Correspondingly fewer dissociation products of the crude gas yield their coldness to the crude gas in the crude-gas exchangers. All heat exchangers can therefore be operated in such manner that the smallest temperature difference occurs at the hot end. Since, however, this is conclusive for the value of the cold losses, the said losses can be kept comparatively low. However, the necessity of having to employ nitrogen at high pressure and the additional exchangers for heating and re heating represents as a whole a comparatively high expenditure of energy and expense for equipment.

it is true that other processes operate without high- 2,895,304 Patented July 21, 1959 ice 2 pressure nitrogen. The necessary degree of coldness must then be generated by expanding the synthesis gas with delivery of external work or the coldness requirements must be met by supplying liquid nitrogen from an airseparating plane. Since, however, the synthesis gas expanded in this way must be compressed to synthesis pressure and the nitrogen from the air separator must in each case be compressed again to the pressure of the washing tower, the complication is not less thanwhen using the high-pressure compressor for nitrogen. By dispensing with the high-pressure compressor, there is also no excess'of specific heat of the gas currents to be cooled over that-of the gas currents to be heated. The temperature differ-4 ences in the exchangers are consequently not substantially smaller at thewarm end'than the mean temperature diiference of the exchangers concerned and the cold losses increase to twice or three times the amount when .the heating surfaces are of the same dimensions. Moreover, it is still essential for the pure nitrogen which is necessary for the washing process and which leaves the air separator at atmospheric pressure to be compressed up to at least the pressure of the crude gas.

With the simultaneous presence of an air separator and a nitrogen washing system, it has also been proposed for the entire coldness production to take place in the air separator and for such an amount of liquid nitrogen to be withdrawn from the latter, compressed and transferred into the nitrogen wash-column that the cold losses there.- of are met. With this working method, the total amount of the fractions to be reheated in the heat exchangers is even somewhat larger than the amount of the gas currents to be cooled; in this way, the cold exchange condi tions are still further impaired for the reasons set forth above. Even if the production of cold in the air separator is somewhat more economical than that with the highpressure nitrogen, this advantage is still not so great that it compensates for the increased cold requirements connected with this working method.

The object of the present invention is a substantial simplification of the nitrogen washing devicewhen this operates in conjunction with a relatively large air-separating plant-and at the same time a substantial saving in energy. The process is advantageously to be employed in connection with a crude gas having a high hydrogen content (higher than88%), such as usually i exists when starting with'a gasifying and splitting opera:

perature, using the nitrogen obtained in an air separator as washing agent and additional gas for the purpose of producing hydrogen-nitrogen mixtures, is characten ised in that the cold nitrogen extracted from the pressure column under the pressure of the pressure column of the air separator is condensed, without intermediate heating and compression, preferably in exchange with vaporising impure washing liquid from the washing column, and is usedfor washing the hydrogen-containing gas mixture in the washing column, which operates at substantially the same pressure as the pressure column of the air separator.

The nitrogen washing is carried out at a pressure which is somewhat lower than the pressure in the pressure column of the air separator (usually 5-5.3 atm.

; This pressure, which is lower than the otherwise usual pressure (10 to 20 atm.), does not then cause any mair separator at the pressure thereof, is transferred in gaseous form in the condition of saturation into the nitrogen washing apparatus, is liquefied in the latter (for example, in heat exchange with evaporating CO+N fraction) and delivered to the washing column. Thus, the initial ammonia cooling is dispensed with as regards the compressor for the pure nitrogen, as well as all exchangers for heating and recooling the nitrogen.

Since the washing column is operated at a somewhat lower pressure than the pressure column of the air separator, it is not possible for any gases to pass therefrom into the air separator. In order to guarantee this, even with an accidental absence of the pressure in the pressure column of the air separator, non-return valves are provided in the nitrogen supply line to the washing column, the valves preventing the hydrogen mixture passing back into the air separator.

With the Working method set forth, a certain amount of pure nitrogen is lacking in the fractions to be drawn off in the heat-exchangers of the air separation plant and the same amount is in excess in the nitrogen washing plant. Compensation is accordingly produced by the fact that some of the dissociation products of the nitrogen washing plant yield up their coldness in the exchanger of the air separation plant.

According to the invention, the hydrogen-nitrogen fraction is chosen for the purpose. If the exchangers of the air separation plant are regenerators, the hydrogennitrogen fraction is conducted through tubes or other shaped heating surfaces which are embedded in the storage mass of the regenerators and render possible a heat exchange of the gas flowing through the tubes with the gases flowing through the storage mass with alternate operation. This avoids any contamination of the hydrogen-nitrogen fraction and the danger that the same volume (within the storage mass) be alternately traversed by air and a combustible gas, so as to set up an explosive mixture of the two gases on changing over the regenerators.

Since the pressure of the air which flows through the storage mass is only slightly higher than that of the hydrogen-nitrogen fraction, only a very small amount of air can pass into the hydrogen-nitrogen mixture, even in the event of possible leakages inside the heat exchangers. The mixture is in any case constantly supervised as regards its purity, so that even traces of oxygen therein are immediately noticed and no dangerous concentration can be set up.

If the heat-exchangers of the air separation plant are not regenerators, but self-purifying exchangers of another type, (for example, so-called reversing exchangers), it is also possible to proceed in analogous manner.

Since the coldness of the pure washed hydrogen residual gas is exchanged in tubes in contact with air, it has become possible for the crude gas also to be cooled in regenerators and in this way for the impurities of the crude gas to be removed by condensation and re-evaporation in the contaminated evaporated washing agent.

With the usual form of the nitrogen washing, this is not possible, because it is practically only pure synthesis gas which is available as the gas to be heated. The synthesis gas is not able to yield its coldness to the crude gas in the alternating reversal operation, because, as a result of the unavoidable leaks in the reversing members and of the flushing operation following the reversal, crude gas containing CO would penetrate into the pure gas, which, as is known, must be absolutely free from C0. However, it is always only possible for a component quantity of the cold-yielding dissociation products to be conducted through heating surfaces which are disposed in the storage mass, while a certain minimum quantity (5( )80% depending on operation conditions) must be conducted through the storage mass, in order that the deposited impurities, such as ice, solid CO and higher hydrocarbons, can be evaporated again. The question is therefore open as to from whence the gas absorbing the impurities is to be taken. The nitrogen being withdrawn from the low pressure column, as the dissociation product containing the least oxygen, is in this case mainly used for flushing the regenerators. It should preferably contain such a small amount of oxygen that no combustible mixtures can be formed when the crude gas regenerators are flushed. As a result of using regenerators for the crude gas, the previous removal of such impurities which become solid in the cooling process is also superfluous. It is only when the CO content of the crude gas is very high that it is expedient to remove this, for example in a pressure water washing operation, until the residual content is 24%, while the usual subsequent liquor washing (fine washing) can be omitted.

Since the air separator must also cover the cold requirements of the nitrogen washing, the amount of air expanded in the turbine is greater than in the case of the independent air separator. In addition, the pure nitrogen necessary for the Washing must be withdrawn under the pressure of the lower column, and is thereby deprived of the liquefaction in the condenser of this column. The re sultant unfavourable refiux condition in the upper column would result in the waste nitrogen leaving the upper column with a considerable oxygen content at the level of 540%. This relatively high 0 content could lead to temporary formation of explosive mixtures with alternate reversal operation with synthesis crude gas.

According to a further feature of the invention, therctore, the air expanded in the turbine or some of this air is not conducted into the upper column, but directly into its own pair of regenerators, where it gives off its coldness to compressed air. It is true that this results in the oxygen yield being somewhat impairedand thus the requirement for compressed air correspondingly increased but at the same time the reflux condition in the upper column of the air separator is substantially improved, so that waste nitrogen with an oxygen content of at the most 3% can be produced without any dilficulties. Since the minimum 0 content in explosive H O mixtures is 3%, any danger of explosion in the synthesis crude gas regenerators is thus eliminated. The same applies as regards the cold periods of the air regenerators in the event of any hydrogen-nitrogen mixtures entering the storage chamber traversed by waste nitrogen, perhaps as a result of leakages in the coil system.

For a better understanding of the invention, and to show how the same is to be carried into effect, reference will now be made to the accompanying drawing, which is one constructional form of an apparatus for separating a compressed gas mixture having a high hydrogen content.

The apparatus used by way of example for carrying the process into effect consists of the following parts: Pairs of regencrators, 1,2, 3,4, 5,6 and 7,8, are provided, the regenerator pair 1,2 serving for the cooling of the crude gas arriving at 48 and the others for cooling the air flowing in at 46. Regenerators 1 and 2 allow heatexchange between crude gas and nitrogen, on the one hand, or residual gas and synthesis pure gas, on the other hand. In regener-ators 3 and 4, air is cooled in exchange with pure nitrogen (in tubes 22,23) and with air expanded in turbines. It is merely the heat-exchange between oxygen and air which takes place in regenerators 5 and 6, through the periodically changed-ovcr storage masses, while in regenerators 7 and 8, there is again such a heat-exchange with air and nitrogen passing through storage masses and pure synthesis gas passing through tubes 24 and 25. The even and odd re generators are changed over in a regular cycle in such manner that each regenerator in a pair always takes over the function of the other member of the pair. The air enters a two-stage air-separating apparatus 9,10 and is expanded in an expansion turbine 11.

It is essentially the crude gas section which is shown in the left half of the drawing. The crude gas to be purified is washed with nitrogen in a washing tower 12, and the washing nitrogen is conducted into the exchanger 13 after initial cooling with synthesis pure gas in the order 14, and is liquefied in this exchanger 13 in tubes 43.

The process which is used in the equipment referred to is composed of the following stages: Crude gas enters at 48 having been compressed to a suitable high pressure of, for example, 5 atm. by a compressor indicated generally at 62, and is cooled in the regenerator 1 (or, after change-over, in the regenerator 2), liberated from water vapor, carbon dioxide and other impurities which can be condensed in the operating temperature range, 1s cooled still further in the exchanger 13 in exchange with contaminated washing liquid which is withdrawn at 16 from the column 13 and expanded in the valve 17 (the washing liquid evaporates between the tubes 43 of the condenser and consists of carbon monoxide and nitrogen); the mixture of gas and condensate is then introduced into the lower part of the washing column 12 which is equipped with filler bodies or bubble plates and in which the impurities still entrained are washed out by liquid nitrogen, which is introduced by way of valve 58 at 18 into the washing column and in countercurrent to the ascending gas. This nitrogen is withdrawn at 19 from the pressure column 9 of the air separating plant, is initially cooled with washed gas in the exchanger 14 and totally condensed in the exchanger 13. The washing column 12 is operated at only a slightly lower pressure than the pressure column 9 of the air separator and the cold nitrogen passes into the exchanger 14 or condenser 13 with practically the same temperature as it had on leaving the pressure column. The condensate, consisting mainly of carbon monoxide and nitrogen, is evaporated at low pressure in the exchanger 13 after having been expanded in the valve 17, and thereafter yields its coldness in tubes 56, 57 to synthesis gas and nitrogen, whereupon the gas discharges at 51.

The air separation plant operates in the following manner: Air enters at 46 having been compressed to a pressure of approximately 5.5 atrn. by means of a compressor indicated generally at 61, and is switched to one member of each of 3 pairs of regenerators, and for example, at the instant illustrated, flows through the regenerators 3, 5 and 7. In regenerator 3, air flows through a heat-storage mass which has been cooled beforehand by exhaust turbine air and in heat-exchange with tubes 22 traversed, by cold pure nitrogen, which has been withdrawn at 19 from the pressure column of the air separator. A heat-exchanger with a storage mass blown cold by means of oxygen only takes place in regenerator 5. The air fraction passing through regenerator 7 exchanges heatwith a storage mass blown cold by nitrogen and by synthesis gas passing through the tube 24. Cold nitrogen from the pressure column constantly flows through the tubes 22 and 23 of the regenerators 3 and 4, and, after being heated, is added through the valves 20 and 21 to the synthesis mixture issuing at 54 so as to amplify the nitrogen fraction. The cold purified synthesis mixture is removed at 28 at the head of the Washing column 12. Some of its cold is already given up in the exchanger 14 to the nitrogen coming from the pressure column of the air separator. It is so distributed to tubes 26 and 27 in the regenerators 1 and 2 and to tubes 24 and 25 in the regenerators 7 and 8, in which it is heated until in the region of the admission temperature of the air before passing through valves 59 and 49 for further use.

The main coldness for the complete installation is produced by the turbine 11, to which fiows the air which has been withdrawn by way of change-over valves (not shown) and distributor valves 29 and 30 at a position between the warm and cold ends of the regenerators 3 and 7. The air for the turbine is always withdrawn from the regenerator which is under pressure, and the regenerator which is not under pressure at the time is disconnected from the air pipe by automatically operated valves which are not shown. Some of the air expanded in the turbine 11 is fed directly into the appropriate re generator (through the regenerator 4 in the illustrated case), and is heated therein, whereupon it discharges at 42, while the remainder of the air is injected into the low-pressure column 10 of the air separator 9, 10, the distribution of the amounts being effected by the valves 31 and 32. The air fraction which is cooled to the lowest temperature in the regenerators 3, 5 and 7 enters the lower part of the pressure column 9 at 33 and is initially split up into impure oxygen and pure nitrogen. The impure oxygen is withdrawn at 34, is. expanded in the valve 35 to the pressure of the low pressure column 10, i.e. to a few tenths of an atmosphere of positive pressure, and is introduced at 36 into the column 10. The pure liquid oxygen 37 is heated in the usual manner by pure nitrogen from the pressure column 9 in the exchanger (condenser) 38. The pure nitrogen which is liquefied in the tubes of the exchanger 38 and flows back into the pressure column 9 is partially collected in the collecting channel 39 and is delivered in liquid form at 41 after initial cooling in the exchanger 15 and expansion at 40. The gaseous nitrogen which is generated and which is withdrawn at 47 passes through the exchanger 15 and in part through the valve 44 to the regenerator 2, in order to be heated in the latter, whereupon it discharges at 52. The other part of this nitrogen is extracted through a valve 45 after the exchanger 15, heated by the regenerator 8 and discharges at 60. In the change-over process, the gases flowing directly over storage masses are in each case changed over to the other members of the pairs of regenerators. The flow then takes place in the direction indicated by the arrows in broken lines. The gas currents conducted in tubes through the regenerators are, on the contrary, always distributed uniformly to both sections.

As a result of the gas currents being conducted in the manner set forth, the crude gas with a high hydrogen content is washed under a pressure which does not exceed the pressure of the air. In addition, the absence of explosive mixtures produced by accidental leaks in the pipe lines conveying combustible gases is ensured, by heating combustible gas in contact with air which is under the same pressure or in contact with nitrogen which contains such a small amount of oxygen that explosive mixtures cannot be formed.

It may happen that the washing column is operated at a somewhat higher pressure than that obtaining in the pressure column 9 of the air separator. In this case, a liquid pump 50, shown in broken lines, is set in operation, automatically to compress the N liquid condensed in the exchanger 13 to the pressure of the washing column 12 and to convey the compressed liquid into the latter column, the non-return valve 58 then becoming operative. At the same time, the modified pressure difference can cause the liquid pump to be switched on. The washing column can of course also be constantly operated at a somewhat higher pressure, if special circumstances require this, in which case the liquid pump operates continuously. The constructional arrangement illustrated in the drawing, can of course be varied, if this should seem expedient. For example, some of the pure nitrogen from the air separation plant can be transferred in liquid form into the nitrogen washing apparatus, if the cold requirements of the latter apparatus cannot be met otherwise. The heat-exchange between the CO-l-N fraction and the crude synthesis gas can also take place with change-over alternating operation. For thispurpose, three synthesis crude gas regenerators may be provided. which may be switched in cyclic exchange to crude gas, waste nitrogen-and CO+N fraction. It is also possible to provide two pairs of synthesis crude gas, regenerators, one pair for the heat-exchange of crude gas with waste nitrogen, and the other pair for the heatexchange of crude gas with the CO+N fraction.

With the combinations of parts of the installation,

some of which contain air or oxygen-containing mixtures, and others combustible gases, great care must be taken to ensure that, even in the event of irregularities in operation and leakages, explosive mixtures which fill relatively large columns cannot be formed. For example, the air-separating section and the nitrogen washing column are arranged in separate housings with the use of insulation. It is also expedient to provide a third housing for the pair of regenerators 7,8, because both air and combustible gas pass through the latter pair. The oxygen content of the impure nitrogen is constantly supervised, and is not allowed to rise substantially above 3%.

The saving in energy which can be produced by the above process is now briefly indicated. It has already been mentioned above that this saving is essentially produced by the omission of the high pressure N compressor and of the ammonia refrigerating machine. Considered thermodynamically, this saving is understandable owing to the lowering of cold losses afforded by the use of regenerators instead of countercurrent heat-exchangers of normal construction for the synthesis of crude gas, and by the omission of the double exchange losses which occur in the heating and recooling of pure nitrogen. ln addition, the production of cold in expansion turbines under the circumstances just described is more favourable than with simple throttle expansion of high-pressure nitrogen (that is to say, without external output), such as must be carried out with independent nitrogen washing plants on heat-balance grounds.

It is, of course, readily possible to use other types of heat-exchangers instead of the regenerators. For example, it is also possible to use tubular-exchangers for the cooling of the crude gas, it this seems expedient, this making it no longer necessary for a substantial part of the synthesis pure gas to be brought into heat-exchange with compressed air. However, the synthesis of crude gas must again be dried completely and freed from CO and other impurities which becomesolid at low temperature.

We claim:

1. Process for the purification and nitrogen-enrichment of a compressed gas mixture having a high hydrogen content, comprising the steps of cooling said mixture in heatexchange with cold, low-pressure nitrogen obtained by a two-stage air separation and with one part of a hydrogenand nitrogen-containing mixture obtained by said purification and nitrogen-enrichment, scrubbing the cooled gas mixture with a washing liquid consisting of substantially pure nitrogen at a pressure only slightly lower than that obtaining in the high-pressure stage of said air separation, thereby to form said hydrogenand nitrogen-containing mixture, reheating the other part of the latter mixture in heat-exchange with air to be separated, condensing substantially pure nitrogen gases from said high pressure stage in heat-exchange with vapourising charged washing liquid consisting substantially of a nitrogen-car bon monoxide mixture to form said washing liquid, using coldexpanded air to cool the compressed air to be separated, expanding part of the incoming air after only partial cooling to form said cold expanded air, feeding the fully cooled remainder to said high-pressure stage, and using low-pressure nitrogen from said air-separation as a flushing gas to remove accumulated impurities deposited by the incoming compressed gas mixture on the cooling thereof.

2. Process according to claim 1, wherein the initial cooling of said compressed gas mixture leads to the substantial removal of water vapor, carbon dioxideand impurities capable of being condensed in the operating temperature range.

3. Process according to claim 1, and further comprising the step of using cold vapours formed by the evapo ration of the charged washing liquid to cool the compressed gas mixture.

- 4. Process according to claim 1, and further comprising the step of further cooling the compressed gas mixture in heat-exchange with vaporising charged washing liquid. 7 v I 5. Process according to claim 1, and further comprising the step of using low-pressure nitrogen from said air separation as a flushing'gas to remove accumulated impurities deposited by the incoming air to be separated on the cooling thereof.

6. Process according to claim 1, and further comprising the step of feeding part or" the expanded air to the lowpressure stage of said air-separation.

7. Process according to claim 1, and further comprising the step of cooling the incoming air to be separated with substantially pure nitrogen gas from said high-pressure stage.

8. A process according to claim 1, and further comprising the step of using cold oxygen from the air-separation to cool part of the incoming air.

9. Apparatus for the purification and nitrogen-enrichment of a compressed gas mixture having a high hydrogen content, comprising a washing column for scrubbing the cooled and substantially purified compressed gas mixture by means of a washing liquid consisting substantially of pure nitrogen flowing in counter-current to said gas mixture, a double column air separator for said two stage air separation, a pair of gas regenerators for cooling the compressed gas mixture in indirect heat-exchange with the low-pressure nitrogen from said air separator, a first pair of air regenerators for cooling air to be separated in indirect heat-exchange with cold expanded air, a second pair of air regenerators for cooling the incoming air in heat-exchange with oxygen from said air-separator, a third pair of regenerators for cooling the incoming air in indirect heat-exchange with low-pressure nitrogen from the upper column of said air separator, turbine means for expanding partly cooled air withdrawn from said first and third pairs of air regenerators, conduit means for returning expanded air to said first pair, a condenser for forming said washing liquid from nitrogenrich gases withdrawn from the top of the lower column, an evaporator within said condenser for evaporating charged washing liquid supplied from the bottom of said washing column, and'tubula-r heat-exchangers within said pair of gas regenerators and within at least one of the air regenerator pairs for heating the cold hydrogenand nitrogen-containing mixture from said washing column, thereby to assist in cooling the incoming crude gas mixture and air respectively.

10. Apparatus according to claim 9, and further comprising tubular heat-exchangers within said gas regenerators for receiving cold vapours from said evaporator, thereby to assist in cooling the incoming crude gas mixture.

11. Apparatus according to claim 9, and further comprising a tubular heat-exchanger within at least one pair of the air regenerators for receiving part of the nitrogenrich gas mixture from said lower column, thereby to assist in cooling the incoming air to be separated.

12. Apparatus according to claim 9, and further comprising conduit means, together with valve means, for diverting part of the low-pressure nitrogen from said air separator to at least one of said air regenerator pairs, thereby to flush the storage mass in said regenerators and to assist in cooling the incoming air to be separated.

13. Apparatus according to claim 9, and further comprising conduit means, together with valve means, for diverting part of the air expanded in said turbine means to about the middle of the upper column of said air Separator.

14. Apparatus according to claim 9, and further comprising heat-exchange coils surrounding 'the tubes of said evaporator for receiving the gas mixture leaving the gas regenerators, to assist in cooling said gases and 'in supplying fully cooled gases to said washing column.

15. Apparatus according to claim 9, and further comprising a heat-exchanger for precooling the nitrogen-rich gases entering the nitrogen condenser in heat-exchange with the cold hydrogenand nitrogen-containing mixture leaving the washing column.

16. Apparatus according to claim 13, and further comprising a non-return valve connected between the bottom of the nitrogen condenser and the top of said washing column, for supplying condensed washing liquid to said column at a pressure slightly lower than that of said lower column, a liquid pump connected between said condenser and said column in parallel with said nonretu-rn valve for raising the pressure of the liquid supply from said condenser to a higher pressure than that in that lower column of said air separator, and means for bringing said pump into intermittent or continuous operation as required for the most desirable pressure in said washing column.

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